This invention relates to methods of fabricating color cathode ray tubes. More particularly, it is concerned with an improved method of fabricating aperture mask assemblies for color cathode ray tubes.
In a color cathode ray tube (CRT), electron beams emanating from the cathodes are directed by means of electromagnetic deflection coils through the holes of an aperture mask onto a pattern of color generating phosphors deposited on the inner glass surface of the tube face panel. In a color television picture tube, for example, the aperture mask may contain upwards of a quarter million holes. Precise registration of the aperture mask holes and the patterned deposit of phosphors must be maintained for proper operation of the tube. This registration becomes particularly critical when thermal strains occur in the mask assembly as the cathode ray tube heats up during operation.
In a typical process of fabricating color CRT aperture mask assemblies, a preformed, apertured mask of thin gauge mild steel is welded to a rigid frame of heavier gauge mild steel. Prior to attaching the mask to the frame, bimetallic mounting members or clips are attached to the periphery of the frame. These bimetallic clips serve the dual functions of providing a means for attaching the mask assembly to the cathode ray tube and of compensating for thermomechanical strains or movement in the aperture mask caused by heating effects during operation of the tube.
During fabrication of a color CRT, the aperture mask assembly is often subjected to humid environments which could cause undesirable rusting of the mild steel. Such rusting is prevented, however, by a step which forms a thin protective coating of black iron oxide on the surfaces of the aperture mask assembly.
A shortcoming of known methods of fabricating aperture mask assemblies occurs in that the step of blackening or oxidizing the mask assembly does not consistently produce an oxide coating that adheres to the bimetallic mounting clips. Because of the differences in oxidation properties of the mask and frame materials from the materials of the bimetallic clips, the conditions for producing a uniform adherent film on the mask and frame are not compatible with the production of an adherent oxide film on the bimetallic clips. In addition, flexing of the bimetallic clips during fabrication or operation of the tube causes flaking of the oxide coating from the clips.
It is therefore the primary object of this invention to provide an improved process for the fabrication of color CRT aperture mask assemblies which produces an adherent film of protective oxide on the bimetallic clips as well as on the mask and frame members of the assemblies.